Open cycle air-conditioners are known in the art and have been based primarily on one system, known as the Munters Environmental Control system (MEC) unit as described in U.S. Pat. No. 2,926,502. As set forth in this patent, the basic open-cycle air-conditioner operates by dehumidification and subsequent cooling of air wherein moist hot air is conditioned by basically a multi-stage process to produce cool air.
In open-cycle air-conditioning systems, a basic multi-step approach is used. Outside air is subjected to removal of moisture through a moisture transfer wheel, with the dried air being cooled by means of a heat exchanger wheel with the subsequent addition of moisture by an evaporative element so as to further cool the air before it enters the area to be conditioned. In the return cycle, the air passes through an exhaust path which includes a further evaporative element, the heat exchanger wheel, a heating element and through the moisture transfer wheel for driving moisture therefrom so as to regenerate the wheel, and is exhausted to the atmosphere.
One of the major advantages of this type of system is that a constant supply of fresh, filtered air is delivered to the space to be conditioned as opposed to the recirculation of air as is found in standard heating and cooling systems.
Known units which have been subject to experimental use provide gas or electrical units in the heating portion of the cycle prior to exhaust through the moisture transfer wheel. Additionally, during the colder winter months, a gas or electrical heating unit may be used in the intake path after the heat exchange wheel for heating the air passing into the area to be conditioned. In this latter mode, the moisture transfer wheel is substantially non-operative.
During recent years, a number of concepts have been proposed for using solar equipment for cooling systems. Among such proposals has been investigations of desiccant air-conditioning systems. However, to applicants' knowledge, no system has been developed which attains a coefficient of performance which would make such systems economically feasible.
The basic principle of the MEC system is that dry warm air can be simultaneously cooled and humidified by contacting it with water vapor. However, in geographic areas where the air is both warm and humid, it must be dried before it can be cooled by evaporation. The efficiency and the effectiveness of an open-cycle air-conditioning system depends upon the ability of the unit to dehumidify the warm moist air input, and upon the effectiveness of the heat exchanger wheel or unit.
Many types of water removal system have been proposed, among which are the rotating moisture transfer wheel which is regenerated in a manner as discussed above. Various desiccants have been proposed for use in this type of wheel. Conventional desiccants used with these wheels are salts, such as lithium chloride, which is used as the drying agent and which is impregnated or sprayed upon the wheel material. Some of the problems involved with the moisture transfer wheels are structural strength, proper balancing and truing and the prevention of "weep". Weep is the condition wherein the salts have a tendency to deliquesce or form aqueous solutions that drip from the wheel. This causes the solutions to either flow out of the unit or to be stripped by the flowing air. When this condition occurs, it leaches the wheel of its absorbent material, substantially reducing its efficiency. With known wheels, this also has a tendency to cause channel collapse or plugging within the wheel. As a result, the amount of the salt used has necessarily been limited to a low level which results in a major reduction in the efficiency of a wheel of given size relative to the removal of moisture from the air.
As to the heat exchanger wheel, operation depends upon the opposite faces remaining at different temperatures. This means that there must be a significant temperature gradient across the wheel in the axial direction. Proposed use of highly thermally conductive material such as metal results in the temperature gradient through the wheel being substantially less, with poor heat exchange and low effectiveness. In the open-cycle air-conditioner system, the heat created by the drying of the air by the moisture transfer wheel must be removed by the heat exchanger wheel. However, migration of the heat axially in the direction of flow of the air through the wheel must be kept to a minimum. If the heat does so migrate, the air stream to be treated exiting from the heat exchanger wheel will not be sufficiently cooled to render the system practical for air-conditioning reasons because the evaporator would not be capable of reducing the higher temperature to an acceptable level of temperature and humidity.
Non-thermally conductive wheel materials such as wax coated asbestos have been previously proposed since they avoid this problem. However, in addition to health hazards, such material presents structural problems relative to balance and truing of the wheel.
Although the thermal gradient across a metallic heat exchanger might be partially counteracted by making the heat exchanger thicker, the increased thickness causes a proportionally increased pressure drop which increases the energy requirement for movement of the required air volume. Additionally, it adds to the weight and bulk of the wheel. To obtain comparable heat exchange in an aluminum wheel of the same design as a wheel which is non-thermally conductive the thickness of the metal wheel would have to be increased due to the difference in thermal conductivity. It has been determined that the maintenance of a suitable temperature gradient across the heat exchanger wheel of an open-cycle air-conditioner is extremely important since the wheel must operate at above 90% effectiveness to provide satisfactory cooling of the treatment stream. A thorough discussion of heat exchangers and the problems inherent therein as relate to the process under consideration is discussed in Compact Heat Exchangers, Kays and London, McGraw Hill Book Company.
Various structural configurations have been proposed for improving the effectiveness of metallic heat exchangers, one such proposal being disclosed in U.S. Pat. No. 3,965,695 issued June 29, 1976 wherein a honeycomb type of construction is proposed. This type of construction requires a considerable amount of metallic material and inherently includes particular manufacturing problems as well as creating a wheel of substantial bulk and weight.
A further problem involved in the open-cycle desiccant air-conditioning system is the problem of temperature transfers due to carry over between the cooling half of the cycle and the heating half of the cycle, that is, the intake air path and the exhaust air path. Also, in the known systems, the amount of heat energy required to regenerate the absorbent material has often resulted in a high heat input rate which leads to a poor thermal coefficient of performance (COP) for the system.
Another problem encountered in known systems is that desiccants such as lithium chloride and lithium bromide become chemically unstable and will deteriorate in the presence of products of combustion such as an open gas flame introduced as the heating element preceding the desiccant wheel in the exhaust path. This is due to the mixture of the combustion products of the gas with the desiccant itself.
Accordingly, it is an object of the invention to provide an improved open-cycle air-conditioning systems with increased coefficient of performance.
A further object of the present invention is to provide an improved system operation of an open-cycle desiccant air-conditioning system.
A still further object of the invention is to provide an improved desiccant wheel for more effectively drying the air intake to the system.
A further object of the invention is to provide an improved heat exchanger wheel so as to substantially increase the effectiveness thereof and, thus, the coefficient performance of the entire system.
A still further object of the invention is to more effectively control the mixture of the air transferred between the desiccant wheel and the heat exchanger wheel in the intake air path.
Yet another object of the invention is to provide a means for substantially reducing the transfer of acqeous solutions within the absorbent material of the desiccant wheel.
Still another object of this invention is the use of a solar energy system integrated into desiccant cooling apparatus.
These and other objects of the invention will be obvious from the following description when taken with the accompanying drawings.